뭐해?

"뭐 해?" (what Mashup answered above) is fine, but if the person is older than you, then you're gonna want to put a 요 on the end...뭐 해요? The second one written there (당신은 무엇을 하고 있습니까?) is, in my opinion, way to literal and formal.

당신은, 무엇을 하고 있습니까?

besides that good answers above, for Present Continuous Tense specifically you can say => 뭐 하고 있어요? [polite spoken form]